The present invention relates to a high intensity discharge lamp (HID). More particularly, this invention relates to a high pressure discharge lamp having increased life and improved lumen maintenance.
Many designs for HID lamps, and in particular high pressure sodium (HPS) lamps, are known in the art. These lamps typically have a quartz, polycrystalline alumina (PCA), or a single crystal alumina (sapphire) arc tube filled with a mixture of gases, such amalgams of sodium and mercury, which form an arc discharge. Sodium and mercury components of the fill material are primarily responsible for the light output characteristics of the lamp. For example, the ratio of the mixture effects the color spectrum of the light output. However, it should be noted that mercury free lamps are also being developed which would benefit from the present invention.
Metal halide lamps of this type are generally comprised of an arc discharge chamber surrounded by a protective envelope. The arc chamber includes the fill of light emitting elements such as sodium and rare earths (e.g. scandium, indium, dysprosium, neodymium, praseodymium, cerium, and thorium) in the form of a halide, optionally mercury, and optionally an inert gas such as krypton, argon or xenon.
It has been found that the life of metal halide lamps is frequently limited by the loss of the sodium portion of the metal halide fill during lamp operation via sodium ion diffusion through the arc chamber. More particularly, fused quartz and alumina are somewhat porous to a sodium ion, and during lamp operation, energetic sodium ions pass from the arc plasma through the chamber wall and condense in the region between the arc chamber and the outer jacket or envelope of the lamp. The lost sodium is then unavailable to the arc discharge and can no longer contribute its characteristic emissions, causing the light output to gradually diminish, and causing the color to shift from white towards blue. In addition, the arc becomes more constricted, and in a horizontally operated lamp, the arc may bow against and soften the arc chamber wall. Sodium loss may also cause the operating voltage of the lamp to increase to the point where the arc can no longer be sustained by the ballast and failure of the lamp may result.
Ceramics doped with MgO (200-1500ppm MgO in the alumina) and used in lamps have been shown to be susceptible to darkening the outer jacket when lamps are operated at wattages above the design space of the ceramic arc tube. Darkening of the glass outer jacket has been linked to a combination of evaporation of the ceramic arc chamber and sodium loss through the walls of the arc tube due to reaction and diffusion mechanisms. This can limit lumen output and the useful life of the lamp. The effect is problematic in newer designs where the sodium-mercury dose weight must be reduced for environmental reasons, or to prevent a cycling phenomenon at end of life.
In addition to the sodium diffusion, the sodium in the arc can react with the alumina at the grain boundaries to form sodium aluminate, which adversely affects the structural integrity of the tube and shortens lamp life. Discharge lamps are being designed for ever increasing internal sodium partial pressure within the alumina arc tube to improve the color rendition and provide a whiter emitted light. However, higher internal sodium pressure further shortens lamp life due to increased rate of sodium loss from the arc chamber. Progressive sodium loss results in a corresponding continual rise in the lamp operating voltage, a decrease of both correlated color temperature and color rendering index, and a color shift from white to pink. Also, the sodium which migrates through the arc chamber wall deposits on the inside wall of the evacuated outer lamp envelope causing a brownish stain on the envelope which, in turn, further reduces the light output of the lamp.
In an attempt to reduce the effect of sodium diffusion through the arc chamber, the skilled artisan has historically relied on coating the arc chamber with sodium diffusion resistant materials. Attempts to solve diffusion problems have included depositing aluminum silicate and titanium silicate layers on the outside surfaces of the arc tube, as described in U.S. Pat. Nos. 4,047,067 and 4,017,163 respectively. Alternatively, U.S. Re. Pat. No. 30,165 discloses applying a vitreous metal phosphate and arsenate coating on the inner surface of the arc tube. In contrast, U.S. Pat. No. 5,032,762 discloses beryllium oxide coatings.
While these methods have met with success in reducing sodium diffusion, the methods also require additional processing steps associated with applying a coating. Furthermore, the lamp""s high temperature of operation, and frequently corrosive environment of use may destroy the adherence between coating and arc chamber substrate. Moreover, cracking and/or peeling can result, exposing the quartz to sodium ions and allowing diffusion to occur. Accordingly, it would be desirable in the art to have a arc tube material which reduces sodium diffusion without the application of additional coatings.
The manufacture of polycrystalline alumina (PCA) and single crystal alumina (sapphire) HPS arc discharge lamps is known. U.S. Pat. Nos. 3,026,210; 4,150,317 and 4,285,732 to Coble, Alaska et al and Charles et al., respectively, disclose the production of a high density alumina body having improved visible light transmission using relatively pure alumina powder and small amounts of magnesia. U.S. Pat. No. 4,285,732 further teaches adding zirconia and hafnia to the magnesia-doped alumina to reduce the chances of precipitating a spinel phase and exaggerated or run away grain growth during sintering.
A need exists for producing an alumina arc tube (PCA or sapphire) having a reduced tendency to permit sodium diffusion and/or binding.
According to an exemplary embodiment of the present invention, a high pressure discharge lamp is disclosed. The lamp includes a discharge vessel having a ceramic wall. Electrodes and a fill are disposed within the discharge vessel. The ceramic forming the discharge vessel wall is comprised of alumina including the following dopants in parts per million:
50xe2x89xa6MgOxe2x89xa61500
100xe2x89xa6HfO2xe2x89xa61500
0xe2x89xa6ZrO2xe2x89xa6700
70xe2x89xa6Y2O3xe2x89xa6300
0xe2x89xa6Sc2O3xe2x89xa61000
0xe2x89xa6Dy2O3xe2x89xa61000
0xe2x89xa6Tb2O3xe2x89xa61000
with the proviso that at least 5 ppm Y2O3, Sc2O3Dy2O33, Tb2O3 or mixtures thereof is included.